Laurentia is an ancient craton which forms the geologic core of North America; it formed between 1. 5 and 1. 0 billion years ago during the Proterozoic eon. The Canadian Shield is the largest exposure of this craton. From the Late Paleozoic to Early Mesozoic eras, North America was joined with the other modern-day continents as part of the supercontinent Pangaea, with Eurasia to its east. One of the results of the formation of Pangaea was the Appalachian Mountains, which formed some 480 million years ago, making it among the oldest mountain ranges in the world. When Pangaea began to rift around 200 million years ago, North America became part of Laurasia, before it separated from Eurasia as its own continent during the mid-Cretaceous period. The Rockies and other western mountain ranges began forming around this time from a period of mountain building called the Laramide orogeny, between 80 and 55 million years ago. The formation of the Isthmus of Panama that connected the continent to South America arguably occurred approximately 12 to 15 million years ago, and the Great Lakes (as well as many other northern freshwater lakes and rivers) were carved by receding glaciers about 10,000 years ago.